This invention relates to electronic devices for receiving a plurality of radiated electromagnetic signals, filtering a selectable channel of frequencies from the received signals, and demodulating the signals of the selected filtered channel. More particularly, the invention relates to television receivers.
Television receivers of the prior art include a radio frequency (RF) section and an intermediate frequency (IF) section. The RF section includes RF filters which are tuned to coarsely filter a band of channels centered about a manually selected channel. The output of the RF filter couples to the input of an RF amplifier. Typically, total gain through the RF section is at least 20 dB to 30 dB. This gain increases the amplitude of signals within the selected channel and additionally makes the noise figure of the system essentially independent of subsequent elements in the receiver. The output of the RF amplifier couples to one input of a mixer, while a second input of the mixer receives mixing signals of a selectable frequency. The selectable frequency is generated such that the selected channel is frequency shifted to approximately 45 MHZ. The output of the mixer couples to a channel selection filter which provides a relatively high impedance path for frequencies outside of the selected channel, and a relatively low impedance path for signals inside the selected channel. Signals at the output of the channel selection filter are therefore primarily comprised of frequencies within the selected channel.
Each television channel contains audio information, video information, and frame synchronizing information. The output of the channel selection filter couples to an audio demodulator which separates the audio information from the selected channel; and the output of the audio demodulator couples to a speaker which generates audible sounds. Similarly, the output of the channel selection filter couples to a video processing unit which separates the video and frame synchronizing information from the selected channel; and the output of the video processor couples to a picture tube which converts the video and frame synchronizing information to pictures.
Prior art television receivers include RF tuned filters before the mixer to insure that image frequencies of the selected channel are sufficiently attenuated at the mixer input so as to not produce interfering mixer output signals. To accomplish this, the bandwidths of the tuned RF filters are only several channels wide, and the center frequency is adjusted to align with the selected channel. RF filters which pass all of the channels of the VHF or UHF band at one time cannot be utilized because image frequencies would destroy reception in the selected channel.
To demonstrate the above point, consider the following. VHF television channels exist from 55.25 MHZ to 71.75 MHZ and from 77.25 MHZ to 87.75 MHZ in the low VHF range, and from 175.25 MHZ to 215.75 MHZ in the high VHF range; while UHF channels exist from 471.25 MHZ to 889.75 MHZ. Also, as is known in the art, the image frequencies of a selected channel lie at 2XIF above the selected channel. Thus, channel 6, for example, with a sound carrier at 87.75 MHZ would have its sound image in a conventional television receiver located at (87.75+90) MHZ. This equals 177.75 MHZ--which is within channel 7. Similarly, the picture carrier for channel 14 is 471 MHZ; and thus its image frequency is 561 MHZ--which is within channel 29. Accordingly, prior art receivers need an RF tuner before the mixer to filter image frequencies.
In the prior art, RF tuning is accomplished by varactor filters, mechanically variable capacitors, etc. However, these are both expensive and difficult to align. By comparison, the television receiver of the disclosed invention has no tuners in the RF section. In one preferred embodiment, a fixed bandpass filter is included which passes the entire low VHF band, a second filter is included which passes the entire high VHF band, and a third filter is included which passes the entire UHF band. These fixed filters are simple in design and eliminate alignment problems. The disclosed invention also includes a mixer having an IF output frequency of between 300 MHZ and 400 MHZ. As a result, image frequencies of the selected channel are placed at least 600 MHZ above the selected channel where they are easily rejected by the fixed bandpass RF filters.
The mixer output of the disclosed receiver couples to a channel selection filter which is implemented by an improved surface wave device (i.e. SWD). Prior art television receivers also used surface wave device channel selection filters. However, the SWD filter of the disclosed invention is an improvement from the prior art in that it is constructed on a relatively small substrate area. The area used by a SWD filter is proportional to its center frequency. Conventional receivers have an IF frequency of 45 MHZ, and thus, the SWD's used therein require substantially more space.
Additionally, the high IF frequency permits the SWD of the disclosed receiver to be implemented in quartz. Quartz has piezoelectric characteristics that are essentially insensitive to temperature changes in the 0.degree.-70.degree. C. range; and thus no temperature compensation circuitry is required. The high IF frequency causes the passband of the SWD filter to be a small percentage of its center frequency. As such, the SWD is most easily implemented with piezoelectric material having a relatively low coefficient of coupling. Quartz has such a characteristic. By comparison, prior art television receivers have a relatively low IF frequency. Thus the passband of the channel selection filter is a large percentage of the center frequency. Thus the channel selection filter requires a piezoelectric material having a relatively high coefficient of coupling. Lithium niobate has such a characteristic and is commonly used. However, the piezoelectric characteristics of lithium niobate are highly sensitive to temperature changes and require temperature compensation.
Another novel aspect of the disclosed system is that the IF section includes two mixers at the output of the SWD filter for frequency shifting the selected channel back to baseband. The first IF mixer frequency shifts the selected channel to approximately 45 MHZ. Most of the gain of the system is then added to the selected channel signals. The second IF mixer is utilized to synchronously detect signals in the selected channel. This architecture permits the above described high IF system advantages to be achieved, without introducing high frequency feedback problems.
Accordingly, it is one object of the invention to provide an improved device for receiving signals in selected frequency channels from a plurality of non-overlapping frequency channels.
Another object of the invention is to provide a television receiver having improved reception.
Another object of the invention is to provide a television receiver having an RF section with no alignment problems.
Another object of the invention is to provide a television receiver having an RF section in which only fixed filters are provded for signal filtering purposes.
Another object of the invention is to provide a telvision receiver having a mixing operation which produces an IF frequency of approximately 300 MHZ-400 MHZ.
Another object of the invention is to provide a television receiver having an IF section with two IF frequencies.
Another object of the invention is to provide a television receiver in which the selected channel is filtered from a plurality of channels by an improved acoustic surface wave device filter.
Another object of the invention is to provide a television receiver in which the channel selection filter has a relatively small substrate area.
Another object of the invention is to provide a television receiver with a SWD channel selection filter that requires no temperature compensation.
Still another object of the invention is to provide a television receiver having a channel selection filter which is embodied on a quartz substrate.